Various types of information terminal devices have been spread to people and things on a global scale. Communication networks, wireless and wired, have been expanding, and the volume of data transferred therethrough has become extremely large. Given such circumstances, the power consumption of information communication systems has become extremely large as well, and intensive studies have been conducted for lowering the power consumption in various layers in information communication networks such as a core network, a metro/LAN, and an access network. One key technique as one of approaches to lower the power consumption of an information communication system is an optical routing technique, in which information processing is performed at an optical signal layer instead of an electrical signal layer. It is considered that routing with optical passive devices by using the optical routing technique can greatly reduce processing at the electrical layers in communication nodes and greatly lower the power consumption of the whole communication network.
One of the important infrastructural elements in an information communication network is a data center. A data center is a general term for buildings designed for the purpose of installing and operating equipment such as Internet servers, data communications, and landline, mobile, and IP telephones. A large number of communication lines are routed into a data center, and a very large number of server computers and the like are gathered inside the building. Reducing the power consumption inside the data center has been a very important issue. In recent years in particular, IP traffic within data centers has been expected to increase greatly due to not only the increase in the number of users but also the increase in the amount of processing over a plurality of servers by the separation of functionality among application servers, data storages, and database servers, the use of distributed processing and parallel processing, and so on. The amount of traffic transferred within data centers has been estimated to be about four times as large as the entire traffic on the Internet. Also, a large proportion of the traffic at a data center is traffic that remains within the data center, and its volume has been said to reach as large as 4.8 zettabyte (4.8×1021) by the year 2015. Now the situation is that the power consumption of a large-scale data center is over 100 MW (100,000 kW). Lowering the power consumption originating from traffic within a data center is a matter of urgency.
In a data center, there are: mice flows, which are small in volume and occur frequently for e-mailing, web searching, and the like; and elephant flows, which are large in volume and occur infrequently for moving virtual machines, data storage, and the like. Thus, for lower power consumption, a hybrid network has been proposed in which mice flows are processed by electrical switches while elephant flows, which account for a large proportion of the traffic, are processed by optical switches.
FIG. 1 is a diagram illustrating an overview of traffic processing in a data center. In a data center 10, a configuration called top of rack (TOR) is employed. A very large number of server computers, storages, and the like are grouped on a rack-by-rack basis, and a plurality of racks 1-1, 1-2, . . . , and 1-n are disposed. With the TOR configuration, a switch (SW) is installed in each of top parts 2-1, 2-2, . . . , and 2-n of the racks, through which the racks are connected to each other, and servers are housed in each rack. This enables equipment management with a high degree of freedom in which one rack is managed as the smallest constituent unit. Besides the server computers and the storages, the data center 10 includes core-layer apparatuses not illustrated, and the core layer and an external network 8 are connected by a large-capacity communication line 7.
Traffic within the data center can be processed such that an electrical SW 3 is used to switch channels 5 between racks for mice flows of traffic between the racks whereas an optical SW 4 is used to switch channels 6 between racks for elephant flows for lower power consumption. Due to the rapid increase in communication traffic in recent years, the number n of racks at a data center is now over 1000, and there is a demand for an optical switch device capable of freely setting and changing the channels between 1000 racks. Specifically, there is a great need for a circuit switching optical switch device of a large scale over 1000×1000.
FIG. 2A to FIG. 2C are diagrams explaining the function and configuration of an optical switch. FIG. 2A illustrates a conceptual configuration of the optical switch. An optical switch 20 operates such that an optical signal input into one of N input ports illustrated on the left side is output to one of N output ports illustrated on the right side. The optical switch 20 can freely form a channel from the TOR part of one rack among the plurality (n) of racks in FIG. 1 to the TOR part of one of the other racks and quickly switches the channel to a different new channel as needed. As illustrated in FIG. 2B, the simplest configuration of an optical switch is a space matrix SW including constituent SW elements 22 arranged in a two-dimensional matrix. A space matrix SW can be configured three-dimensionally by using MEMS (Micro Electro Mechanical Systems). However, space matrix SWs, which include constituent SW elements, are known to be such that as the number N of ports increases, the required number of constituent SW elements 22 increases in proportion to N2, as in a conceptual diagram of the correlation between the number N of ports and the number of constituent SW elements illustrated in FIG. 2C. For this reason, in a situation where the number of server racks connected to each other in a data center is more than 1000 and the numbers N of input ports and output ports of the optical switch device are each more than 1000, the number of constituent SW elements will be very large, and therefore a space matrix SW will not be realistic in view of the circuit scale and the cost.
By employing a multi-stage configuration in which a plurality of matrix SWs are cascaded, the number of constituent SW elements in the whole SW can be reduced to some extent. Nonetheless, its effect is limited. As the number N of ports increases, the required number of constituent SW elements 22 increases in proportion to N1.5, as illustrated in FIG. 2C. In the case where the number N of ports of the optical switch device is over 1000, an increase in the number of constituent SW elements is still a major problem even if the multi-stage configuration is employed. Further, configurational restrictions will also be added in order to satisfy requirements for a complete switch. A wavelength routing SW has been known as a space optical switch configuration with which the increase in the number of constituent SW elements with an increase in the number N of ports can be suppressed to be substantially linear (the first power of N), as illustrated by the dotted line in FIG. 2C.
FIG. 3 is a diagram conceptually illustrating the configuration of a wavelength routing SW. An exemplary wavelength routing SW 30 is a space SW that switches 100 inputs to 100 outputs and includes 100 wavelength-tunable light sources (LDs: laser diodes) 31-1 to 31-100 on the input port side and a demultiplexer 35 having 100 output ports 36-1 to 36-100 on the output port side. Each of the 100 wavelength-tunable LDs can be set to one of different wavelengths λ1 to λ100, and modulate light beams to be output from the LDs according to information signals 32-1 to 32-100, respectively. The modulated light beams with the different wavelengths from the wavelength-tunable LDs 31-1 to 31-100 are multiplexed by a coupler 33 and amplified by an optical amplifier 34 as needed. The multiplexed modulated light beam from the optical amplifier 34 is demultiplexed by the demultiplexer 35 for the 100 output ports 36-1 to 36-100, which are assigned with the wavelengths λ1 to λ100. As the demultiplexer 35, an arrayed waveguide grating (AWG) can be used, for example, and the demultiplexer 35 can demultiplex a wavelength-multiplexed light beam with at most 100 different wavelengths λ1 to λ100 by wavelength.
A wavelength routing operation can be described as below. For example, the oscillation wavelength of the wavelength-tunable LD 31-1, corresponding to the first input port, is set to λ100. Here, the output light beam with the wavelength λ100 is modulated by the information signal 32-1, which has been input into the first input port. The modulated optical signal with the wavelength λ100 is output by the demultiplexer 35 to the output port 36-100, which is the 100-th output port. In other words, the information signal input into the first input port is connected to the 100-th output port. Here, if the oscillation wavelength of the wavelength-tunable LD 31-1 is set to λ1, its light beam will be output to the output port 36-1, which is the first output port of the demultiplexer 35. Similarly, if the oscillation wavelength of the wavelength-tunable LD 31-1 set to λ50, its light beam will be output to the output port 36-50, which is the 50-th output port of the optical demultiplexer 35.
By freely setting the oscillation wavelengths of the 100 wavelength-tunable LDs in the above manner, it is possible to freely select the ports to which to output the modulated optical signals. Since it is possible to output 100 different information signals to any positions among the 100 output ports, a 100×100 wavelength routing SW is obtained. While a wavelength routing SW completely differs from a space matrix SW in the configuration of the constituent SW elements due to their difference in principle, the hardware scale of the circuit increases substantially in proportion to the number N of input ports/output ports. It has therefore been considered possible to obtain an optical switch device with smaller scale hardware and a smaller cost than a space matrix SW.
In optical communication, the C-band (approximately 4400 GHz in width) has been widely used for its small propagation loss and has been available for a wide range of related devices and components. It is possible to use about 100 communication channels (100 waves) by setting the bandwidth of a single communication channel to 50 GHz in this C-band and also using a band somewhat higher than the C-band. With the bandwidth of a single communication channel halved to 25 GHz, it will be possible to form 200 channels (200 waves) within the C-band, yet the bandwidth of an information signal will be halved as well. There will also be a problem in the accuracy of the wavelength control on related components. Hence, it is not easy to increase the number of ports of a wavelength routing SW to 100 or more. To address this, a configuration in which wavelength routing units are arranged in parallel has been proposed as a configuration for further increasing the scale of a wavelength routing SW.
FIG. 4 is a diagram illustrating the configuration of a wavelength routing SW in a conventional technique having wavelength routing units arranged in parallel. In a wavelength routing SW 40 in FIG. 4, unlike the simplest wavelength routing SW configuration in FIG. 3, K wavelength routing units 48-1 to 48-K are arranged in parallel at the subsequent stage. On the input port side, modulated light beams from KN wavelength-tunable LDs 41 divided into N groups are selected and combined by N delivery and coupling (DC) switches and supplied to the K wavelength routing units 48-1 to 48-K. By arranging the wavelength routing units 48-1 to 48-K in parallel, it is possible to provide many ports in the optical switch device without increasing the number of wavelengths set for the wavelength-tunable LDs 41 on the input port side. Note that each of the DC switches is also called a multicast switch, and various products with different numbers of input ports and output ports are available as general-purpose products with M×N ports.
FIG. 5 is a diagram illustrating a configuration example of the DC switches used in the wavelength routing SW in the conventional technique. A DC switch 50 represents an example of the minimum configuration. It is configured to select and combine three waves (λ1, λ2, λ3) from three wavelength-tunable LDs and includes three 1×3 switches 51-1 to 51-3 and three 3×1 optical multiplexers 52-1 to 52-3. FIG. 5 illustrates a setting example where the three wavelengths λ1, λ2, and λ3 into the three input ports of the DC switch 50 are all output to the first output port 53. With the DC switch 50, each of the three wavelengths can be output to any of the output ports. Note that if the DC switch in FIG. 5 is used in the opposite signal direction, the DC switch will operate to demultiplex a multiplexed light beam (λ1, λ2, λ3) and select and output only a light beam with one of the wavelengths. The switch is called a delivery and coupling (DC) switch as its operation includes the above. A method of using DC switches in which the direction of optical signals is opposite to the direction illustrated in FIG. 5, as mentioned above, will be discussed in later-described embodiments of the present disclosure.
In FIG. 4, one of the wavelengths for the first group of wavelength-tunable LDs can be set to select and be connected to one of the K wavelength routing units 48-1 to 48-K by a DC switch 42-1, and a channel to any one of the KN output ports on the output side can be formed. The inventors have proposed a specific configuration example of a wavelength routing SW (NPL 1) based on the configurations illustrated in FIG. 3 and FIG. 4 in an attempt to obtain a larger-scale wavelength routing SW that can handle increase in traffic in a data center.
FIG. 6 is a diagram illustrating the configuration of a large-scale wavelength routing SW in a conventional technique proposed by the inventors. A wavelength routing SW 60 illustrated in FIG. 6 is a more specific representation of the whole configuration in FIG. 4 with the DC switch configuration in FIG. 5. It includes wavelength-tunable LDs divided into N groups and M wavelength routing units 63-1 to 63-M arranged in parallel. FIG. 6, though omitting specific description of the configuration and operation, illustrates an example of an 800×800 large-scale wavelength routing SW with good transfer signal characteristics obtained by using 8×8 DC switches, erbium-doped fiber amplifiers (EDFAs), non-cyclic AWGs.